package firstExercise;

import java.util.Scanner;

public class one_two {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        double length=sc.nextInt();
        double a=length*length*1.00;
        double r=length/2.00;
        double b=3.14*r*r;
        System.out.printf("%.2f",a-b);
    }
}
